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J. van Overbeek and L. Dowding 



disinfection, it was found that microorganisms will grow on the leaves, 

 and when this happens, growth is abnormal and inhibited. The addi- 

 tion of a low concentration of isopropyl alcohol to the test solutions 

 has also been used to keep them relatively free of interfering micro- 

 organisms. The Clorox-dipped sections are then jjlaced in 50-ml. 

 beakers containing 2 ml. of test solution. The beakers are covered 

 with watchglasses and placed on a slow shaker for 48 hrs. All of these 

 operations have taken place in the physiological darkroom (Figure 2). 



The objective of this new test was not so much to assay extracts of 

 plant material for their gibberellin content, but rather to provide a 

 convenient means of studying gibberellin action. We want to report 

 here on a striking inhibition of the effect of gibberellic acid (GA) by 

 other regulators, specifically indole-3-acetic acid (lAA) (Figure 3). 

 Because inhibition of kinetin by auxin has been reported in the liter- 

 ature (5), kinetin was included in some of the tests.. As the results in 

 Figure 4 show, kinetin does inhibit the effect of gibberellic acid at the 

 same low concentrations at which indole-3-acetic acid is effective. 

 While lAA by itself inhibits growth, kinetin alone appears to promote 

 growth. 



When low concentrations of indole-3-acetic acid are applied to- 

 gether with gibberellic acid, the effect of the latter is much sup- 

 pressed (Figures 3 and 4). The entire GA curve is dropped. This in- 



o 



iLl 



% 



Fig. 3. Avena leaf growth from basal coleoptile sections as inlliuiucd 1)\ low con- 

 ceniralioiis of gibberellic acid (GA) and indolcacetic acid (lAA). Gibberellic acid 

 promoted leaf growth, while indoleacetic acid inhibited it. Throe tests were run 

 on consecutive days; each line is the average of 21 sections. 



